Power amplifiers have become smaller and smaller lately. This is not really a surprise as manufacturers are trying to incorporate multiple amplifiers into the same product. Part of the reason for amplifiers become smaller is the ongoing integration of discrete components into semi conductors. Especially for amplifiers which are often used, spending the extra money for creating an integrated circuit often makes sense.
Another reason for the shrinking size of power amplifiers is the fact that the efficiency has increased rapidly. This is because there has been a fundamental shift in technology. New amplifiers almost exclusively are switched mode amplifiers which means that power efficiency has increased to upwards of 90%. This as a result has lead to the reduction of the size of heat sinks which are required to ventilate excessive heat. Obviously, amplifiers which have low power efficiency will generate a vast amount of heat. Also, tube amplifiers are rather inefficient and require a lot of ventilation.
In addition to the amplifier requiring less ventilation, the power supply which is a crucial element also can become smaller. For a given wattage, the amount of energy that has to be supplied to the amplifier is less as power efficiency goes up.
Nowadays you can find amplifiers which are tiny. Some are no bigger than a deck of cards. Those amplifiers are ideal for distributed installations. A distributed installation means that every loudspeaker has its own dedicated power amplifier. This is quite similar to using active speakers where the power amplifier is already built into the speaker enclosure. Distributed installations make a lot of sense because you don’t require one bulky power amplifier. Further, you don’t need large speaker cables because the distance between the amplifier and each speaker is rather short.
However, keep in mind that in these situations every loudspeaker has to be located near a power outlet. That is because no power amplifier works without being supplied with ample power. Also keep in mind that the wattage of the amplifier should be quite a bit more than the wattage that you will actually require. This is because most amplifiers run into large amount of distortion once they hit their rated wattage. As a safe bet, you should never exceed 50% of the rated wattage in order to enjoy clean audio. Therefore, taking advantage of the shrinking size of amplifiers you should be able to afford an amplifier that fits into your space and offer sufficient wattage.